This invention relates generally to gas turbine engines, and more particularly to passive clearance control systems between components of rotating air seals. Gas turbine engines operate by combusting fuel and compressed air within a combustor to create heated gases with increased pressure and density. The heated gases are used to drive a turbine that turns rotor blades inside a compressor section of the engine, which provides the compressed air used during combustion. The turbine or a second turbine is also typically used to produce rotational horsepower, which can be used to turn a fan to produce thrust, or to turn a generator to produce power. Compressor air is also used to maintain pressure and thrust balances within the engine, or to direct cooling air to various hot sections of the engine. Thus, bleed air is siphoned off the compressor section and directed to other various portions of the engine where it can perform the desired function. Gas turbine efficiency is, therefore, closely linked to the ability of a gas turbine engine to direct air flows within the various engine sections efficiently and without leakage. As such, various air seals are used throughout the engine to maintain air flows and pressure balances.
For example, it is particularly advantageous to maintain the mass flow from the compressor section to the combustor and on to the turbines in order to maintain mechanical and thermal engine efficiency and fuel economy. Compressors and turbines are comprised of alternating stages of vanes and blades that are arranged radially around a center axis to form an axial flow path. The blades are fixed at their inner end to rotating rotors connected to a turbine shaft, and the vanes are suspended from engine casings between the rotating blades. Thus, the rotating blades come into close proximity with the stationary engine casings at their outer end. Conversely, the stationary vanes come into close proximity with the rotating rotors at their inner end. The gap between these rotating and stationary parts, albeit small, permits air to leak out of the flow path, thus reducing the efficiency of the engine. Therefore, in addition to sealing between engine sections along the main flow path, it is necessary to seal the flow path at both the inner and outer radii of the vanes and blades.
Previous attempts to seal the gaps between these and other rotating components include the use of knife edge, labyrinth and brush seals. However, it is difficult to maintain the tight tolerances required with these sealing arrangements due to deformations that the engine undergoes during various stages of operation. For example, during high output requirements of the engine, due to extreme heat conditions, the engine casings grow in diameter due to thermal expansion. Additionally, the turbine shafts grow in length due to thermal growth resulting from the high temperatures reached during operation of the engine. As such, it is difficult to maintain alignment and clearance height between the various seal types, reducing their effectiveness. Thus, a fixed clearance height is typically settled upon that functions adequately for all engine operating conditions, but optimally for few, if any. Solutions to this problem have involved active clearance control systems, which are complex and difficult to accurately control. Therefore, there is a need for an improved clearance control system for components of rotating seals.